This invention relates to sonar transducers and more particularly to a flextensional transducer. There exists a need in underwater acoustics for an efficient light-weight, small relative to a wavelength, high power, high duty cycle, very low frequency source for single element or line array applications. The transducer types which are presently available are hydro-acoustic sources, variable reluctance transducers, hoop mode ring devices, and flexural mode transducers. The first three of these transducer types are considered to be less desirable for various reasons than the flexural mode transducers.
Although there are many types of flexural transducers, the flextensional transducer has been demonstrated to be exceedingly useful for low frequency applications. FIG. 1 shows an isometric view of a prior art Class 4 flextensional transducer 10. The common characteristic which makes all flextensional transducers efficient radiators of power at low frequencies is large displacement over a relatively large surface area. The elliptical geometry of the shell 11 is such that it provides a lever-type action which amplifies the displacement of the flattened (diaphragm) region 12 of the shell 11 resulting from the longitudinal movement of the ends 13 caused by the electrical energization of the ceramic drive stack 14 produced by alternating current energization of the wires 15 from a source (not shown). The mechanical transformer effect which converts small longitudinal motion of the ends 13 into a relatively large transverse motion of the diaphragm regions 12 also results in enhanced compliant and inertial loading enabling the flextensional transducer 10 to achieve a low resonance frequency in a very light-weight package. In addition, flextensional transducers which have piezoceramic drive stacks 14 typically have efficiencies in excess of 70%. The interior of the flextensional transducer 10 is maintained water-tight by cover plates 15 which are held in compression against rubber gaskets 16 by bolts 17 threadedly connected into the threaded holes 18 of support 19 to form a water-tight enclosure. Additional supports 19 may be used to compress cover plate 15 against gaskets 16 to allow a thinner cover plate 15 to be used while continuing to provide a water-tight interior. The interior of the transducer 10 is at normal atmospheric pressure when assembled and made water-tight.
The conventional air-backed flextensional transducer 10 is, however, limited in certain respects:
The mechanical compressional prestress applied to the ceramic drive assembly 14 in order to achieve high power operation is supplied by means of a force F which compresses the shell 11 at the diaphragms 12 thereby extending the space between the ends 13. Insertion of shims 21 between the shell ends 13, the support 19, and the drive assembly 14 and release of the force F causes the shell 11 to relax toward its non-prestressed state thereby compressing the drive assembly 14. During deployment, as the operational depth and hence the hydrostatic pressure of the water in which the flextensional transducer is immersed increases, the shell 11 is flattened by the force of the water pressure on the diaphragms 12 and the mechanical prestress is diminished. This results in a degradation in acoustic output power due to stress limitations with increasing water depth and severely limits the transducer 10 depth capability. Fluid and compliant tube pressure compensated device designs have been utilized to attempt to offset this effect, but these designs have all suffered from severely reduced efficiency due to excessive viscous losses.
The prior art air-backed design of FIG. 1 is frequently thermally limited to short pulses and duty cycles not exceeding 10 or 20% because of the difficulty in removing heat from the interior-mounted drive assembly 14.
While greater ellipticity (smaller spacing between diaphragms 12 relative to the ends 13) would result in greater displacement amplification and power output, ellipticity is restricted in the design of FIG. 1 because of the requirement to have sufficient space within the shell 11 to accomodate the drive assembly 14.
Since both bandwidth and resonance frequency are directly proportional to shell thickness and inversely proportional to shell circumference, the thick shell of the prior art transducer 10 which is required to maintain mechanical prestress results in a loss of bandwidth and an increase in shell longitudinal and transverse dimensions to keep down the resonance frequency.